ECG-Derived Respiration

Several techniques can be used to obtain a respiration signal from an
ECG. A popular technique used in bedside patient monitors involves
measuring transthoracic impedance using the ECG electrodes. Unlike
those described below, this method obtains the respiration signal from
the ECG electrodes rather than from the ECG signal.
It requires special-purpose hardware and is not suited to recovering
respiration from a recorded ECG, but it has the significant advantage
(relative to the other methods mentioned here) that a continuous
respiration-related waveform can be obtained.

Another well-known method is based on observing the beat-to-beat variations in
RR intervals or their reciprocals, which are primarily due to respiratory sinus
arrhythmia (RSA) in most individuals. This method works best in young, healthy
subjects, in whom RSA is most pronounced.

The EDR (ECG-Derived Respiration) technique is a third approach. It is based
on the observation that the positions of ECG electrodes on the chest surface
move relative to the heart, and transthoracic impedance varies, as the lungs
fill and empty. Thus the lead axes vary at different points in the respiratory
cycle, and any sufficiently precise measurement of the mean cardiac electrical
axis shows variations that are correlated with respiration. This method is
particularly effective if two or more ECG signals are available, but a single
lead is sufficient. The EDR can be obtained even in the context of congestive
heart failure, in which RSA may be absent.

None of these methods supplies a calibrated respiration signal; for tidal
volume and air flow measurements, other techniques (spirometry, measurements
from nasal thermistors, and plethysmography) are best. Nevertheless, any of
these methods can provide a basis for estimating respiration rate and for
detecting apneas.